1. Field of the Invention
This invention relates to an insulating bushing of the type wherein a support elastic member is interposed between inner and outer cylinders and formed with fluid chambers exhibiting a vibration damping effect, and more particularly to such an insulating bushing through which a vibratory body such as an internal combustion engine is supported on a vehicle body, the vibratory body generating both vibrations within a low frequency and large amplitude region and within a high frequency and small amplitude region.
2. Description of the Prior Art
In general, a power unit constituted of an engine and a transmission is supported through power unit mounts (engine mounts) on a vehicle body in order to prevent engine vibration and the like from being transmitted to the vehicle body. Such a power unit mount usually includes an elastic support member made of rubber or the like and securely put between two attachment members which are respectively connected to the vehicle body and the power unit. However, with such a power unit mount, there is possibility of the elastic member being broken and cut into two by any cause. In order to overcome this difficulty and to make the power unit mount small-sized, an insulating bushing of so-called inner-outer cylinder type has been proposed and put into practical use in which an elastic support member is securely disposed between inner and outer a power unit and a vehicle body, while the outer cylinder is connected to the other of them. With the thus configured insulating bushing, the load of the power unit is supported by the elastic member, and the inner cylinder is prevented from getting out of the outer cylinder even if the elastic member is broken and cut into two because the inner cylinder is surrounded by the outer cylinder.
Such an inner-outer cylinder type insulating bushing is disclosed, for example, in Japanese Patent Provisional Publication No. 61-65935. In this insulating bushing, the elastic support member between the inner and outer cylinders is formed with two fluid chambers which are located opposite with respect to the inner cylinder and communicated with each other through an orifice formed in a block securely mounted on the inner cylinder. Accordingly, when relative displacement between the inner and outer cylinders is made along with vibration of the power unit, the elastic support member deforms so that one fluid chamber expands simultaneously with contraction of the other fluid chamber, thereby causing the fluid within the fluid chambers to move through the orifice. This damps vibration of the power unit.
Now, in order to set target vibrations to be damped within a low frequency and large amplitude region, it is required to minimize the spring constant due to expansion elasticity of the fluid chamber or to increase the mass of liquid within the fluid considered to enlarge the cross-sectional area of the orifice for communicating the fluid chambers or to increase the length of the orifice.
However, the orifice in the insulating bushing as disclosed in Japanese Patent Provisional Publication No. 61-65935 must be formed straight because of being formed in the block mounted on the inner cylinder, so that the cross-sectional area and the length of the orifice are small. Consequently, it is very difficult to set the target amplitude region, thus making it impossible to damp engine shake and idling vibration. Additionally in the small cross-sectional area orifice when high frequency and small amplitude vibrations are transmitted to the insulating bushing. Thus, it is also impossible to tune the target amplitude region, thus making impossible to damp high frequency and small amplitude vibrations.